I am still looking for a proportional font where I can clearly distinguish to single quotes from a double quote, and distinguish a single quote followed by a double quote from the double followed by a single. Do you have some suggestions?

It makes code indentation much more clear. Though what are the advantages of using non mono for coding? Are there clear advantages for that?

If you are only indenting the start of your lines (and not inside the lines), a proportional font is actually completely unproblematic.

For IDE i can't speak but, they're necessary for terminal/textual ui's like vim, emacs, kakoune, helix, lazygit, etc.